Just a thought: I wonder if population growth is actually realistic in the game? A non hostile world can support a maximum of 11 billion at antimatter tech level, and if we're raising it from say TL1 it would take less than 1 week to reach that population cap (after building the appropriate arcologies etc etc). In comparison, Earth's population growth is around 80 million people per year. Assuming 1 "cycle" is the same as 1 Terran year, the world populations in Anacreon are increasing at rates much too high to be realistic. Maybe just nitpicking too much, or perhaps people in the 4th millennium are just very very, err... fertile. And there could be cloning tech or something for all we know...

"Live long and may the Force be ever in your favour, Mr. Potter"
-- Gandalf (The Chronicles of Narnia)

This is an issue that a lot of 4x games have, and I think a lot of the time it just has to be handwaved away. GalCiv II explained it as "not everyone on the planet belongs to your civilization and they voluntarily join/leave", which may or may not be an appropriate way to think about Anacreon. Since space travel is ubiquitous I think that interstellar emigration from other worlds, natural population growth, and accelerated maturity are probably "good enough" explanations.

The "empires" in Anacreon may be analogous to medieval Venice and other naval empires, which dominated their own sphere but could not effectively control territory inland because their navies were unable to intimidate them. "Inland" in Anacreon might be a large pool of people in each system who are able to emigrate onto and off of planets when there is an opportunity to make a livelihood but who would not normally live on them, preferring to live on indefensible asteroids, moons, space stations, economically irrelevant planets that would not be of interest to an interstellar empire and are thus not modeled in the game, etc.

We see warp and jumpdrives depositing fleets right in orbit of planets: maybe there is only ever one planet in a system can be easily accessed in this method, but then it takes months to traverse distances within the system using chemical rockets and only small cargoes like people can be carried economically, not millions of tons of resources. Other planets and inhabited regions of a solar system thus can't be easily accessed or contribute to trade and aren't relevant to an interstellar empire, but they are a source of population when planets are expanding and a sink for the populace to get displaced to when it becomes too expensive to keep living on high-tech worlds (we see population decrease as TL increases above TL 5-6).